Stainless steel is often treated to render the surface inert to corrosion. In the past, the surface treatment process known as passivation was actually a two-step method that today is differentiated into the two separate processes. Pickling, or descaling, is done to remove scale and to clean the surface. The passivation process takes place following the pickling process in order to provide the surface with desirable characteristics, often including the formation of a protective oxide film coating to prevent interaction with the exterior environment.
While the term passivation is generally used to imply formation of an inert or robust surface, the particular characteristics required for passivation can vary with intended use and levels of quality and can be done using different standards and proprietary methods to achieve various goals. For instance, the term can apply equally to a process that merely renders the surface inert by formation of a thin oxide layer as well as to electrochemical passivation as it occurs during cyclic voltammetry. In applications that involve storage or other contact with gases, and particularly those involving hydrogen isotopes, surface passivation requires that not only should the surface be inert to surface catalytic action, but it should also resist hydrogen outgassing and absorption.
To improve handling and storage of hydrogen isotopes, a stainless steel passivation process has been developed (termed “Q-passivation” where “Q” is a stand-in for the hydrogen isotopes of H, 2H, and 3H, protium, deuterium, and tritium) that includes electropolishing and vacuum treatment with associated secondary operations (e.g., pickling). Q-passivation can smooth the surface, remove residual hydrogen, and create a chromium and chromium oxide rich surface. An example of a typical Q-passivation technique is described by Sasaki (J. Vac. Sci. Technol. A 25(4), July/August 2007). In the process presented by Sasaki, electropolishing of the surface was performed followed by a vacuum heat treatment to 400° C. It is discussed in this example that the reduction of surface roughness in turn reduced available atomic surface area for hydrogen out-gassing. Q-passivation processes can form a chromium oxide enriched layer that can inhibit the migration of hydrogen liberated from the bulk stainless steel and can also limit surface interaction by diminishing catalytic metal centers on the surface that can react with hydrogen and water. Unfortunately, such methods have exhibited variable and often marginal results, particularly when considered for use with containment vessels, manifold components, etc., for the storage, conveyance, preparation, and characterization of tritium gas. Tritium storage and conveyance is particularly problematic as tritium is not only radioactive but is also highly reactive with many materials. It is easily adsorbed onto and absorbed through the surface of containment vessels. Interaction of tritium with the surface can modify the contained gas composition by isotopic exchange as well as by reaction with surface elements and adsorbed gas species.
Accordingly, what is needed in the art is a process for treating stainless steel that can better render the surface inert to hydrogen out-gassing and inhibit reaction of the substrate with hydrogen and water. Such a process would be highly beneficial for treatment of gas standard bottles and equipment for storage and conveyance of hydrogen isotopes and in particular for tritium. The resulting stainless steel surface could be inert to tritium, minimize isotope exchange between H2 and D2 and minimize catalytic breakdown at the surface.